Diarylide yellow pigments are well known in the art. These pigments are particularly useful as the coloring components of various types of printing inks. Diarylide yellow pigments are organic compounds possessing a disazo structure, wherein both of the azo linkages are couplings with acetoacetarylides. The diarylide yellow pigments are typically manufactured by tetrazotizing 3,3'-dichlorobenzidene and forming the tetrazonium salt, 3,3'-dichlorobenzidine tetrazonium chloride. The tetrazonium salt is then coupled with an acetoacetarylide such as aceto acetanilide to form the coupling product.
Diarylide yellow pigments are classified according to the acetoacetarylide to which the tetrazonium salt of 3,3'-dichlorobenzidine is coupled. AAA diarylide yellow is the product of 3,3'-dichlorobenzidine coupled into acetoacetanilide (AAA). AAMX diarylide yellow is the product of 3,3'-dichlorobenzidine coupled into acetoacet-2,4-xylidide (AAMX). AAOA diarylide yellow is the product of 3,3'-dichlorobenzidine coupled into acetoacet-o-anisidide (AAOA). AAOT diarylide yellow is the product of 3,3'-dichlorobenzidine coupled into acetoacet-o-toluidide (AAOT). HR yellow is the product of 3,3'-dichlorobenzidine coupled into acetoacet-2,5-dimethoxy-4-chloranilide. Various types of diarylide yellow pigments can be manufactured depending upon the acetoacetarylide coupling agent or agents used, and, the compounds will have varying color and pigment characteristics.
Use of a different type of coupling agent, a pyrazolone derivative, gives redder pigments: 3,3'-dichlorobenzidine coupled into 3-methyl-1-phenyl-2pyrazolin-5-one (PMP) gives PMP diarylide orange.
Redder diarylide pigments are made also by using another benzidine derivative, 3,3'-dimethoxybenzidine: when this is tetrazotized and coupled into AAA, dianisidine orange results; when coupled into PMP, dianisidine red results.
Diarylide yellow pigment compositions are the primary organic yellow pigment compositions produced in the United States. Of the diarylide yellow pigment compositions, AAA diarylide yellow is the most widely used.
As previously mentioned, one of the primary applications for diarylide yellows is as a coloring agent in printing inks. The diarylide yellow pigments can be used in oil based, water based, or solvent based ink compositions. Depending on the ink composition, these pigments are either left as is or treated with resins, surfactants, etc., a more complete description is contained in "Surface Treatment of Organic Pigments for Printing Ink Applications", by B. G. Hays, in "American Ink Maker," Vol. 62, No. 6 (June, 1984), pp 28-50), which is incorporated by reference. These treatments usually improve various printing ink and printing process properties.
In order to use or incorporate diarylide yellow pigments in publication gravure printing inks, it is typically necessary to pretreat the pigments so that they become easily dispersible as described in "Criteria for Pigment Selection for Publication Gravure Inks," by B. G. Hays, in "Gravure Technical Association Bulletin," Vol. 34, No. 4 (Winter, 1983), pp32-39), which is incorporated by reference. Typically, the diarylide yellow pigment is made easily dispersible by treating the pigment with a fatty alkyl primary amine. This is typically done by adding an acidic solution of the fatty alkyl primary amine to a diarylide yellow pigment, raising the pH to precipitate the amine, filtering, washing, drying the presscake and forming the ketimine between the amine and aceto groups on the surface of the acetoacetarylide pigment after the pigment reaches less than about 1% water content. The time and temperature required for this ketimine reaction necessitate using fatty alkyl amines with low volatility. A typical ketimine reaction is as follows: ##STR1##
The resulting treated pigments, although easily dispersible, are coated with a substantial amount of residual, unreacted amine. This unreacted amine is removed with considerable difficulty by drying at elevated temperatures and presents several disadvantages. Firstly, the unreacted amine can react with publication gravure ink limed rosin vehicles, resulting in inks that penetrate into and hold out poorly on uncoated papers and give weak prints. Secondly, if the pigment is incorporated into an oil-based offset ink, the unreacted amine can emulsify excessive amounts of acidic fountain solution into the ink, thereby producing a variety of printing problems such as tinting and scumming.
It is also known in the art that many diarylide yellow pigment compositions, while having desirable color strength, degrade in the presence of heat, elevated temperatures or light or may be too costly (e.g., HR Yellow). The choice of a particular diarylide yellow pigment is dictated by the requirements of the particular application, e.g., light fastness, heat resistance and economics.
There is a constant search in this art for diarylide yellow pigment compositions having improved properties such as improved color strength, improved transparency and improved resistance to degradation. It is known in the art that the fatty alkyl amine treatment of diarylide yellow pigment, with the resulting residual free amine, results in degradation of the printing ink vehicle. It is also known that the excess unreacted amine results in excessive emulsification of fountain solution in oil-based offset printing inks. It is also known that heat treatment to remove the free amine can adversely affect the pigment characteristics.
Accordingly, what is needed in this art are easily dispersible diarylide yellow pigment compositions, having the advantages of the amine-ketimine treatment, but containing no residual unreacted amine and a method of producing such compositions.